Tailoring product characteristics in the carbonisation of brewers’ spent grain through solvent selection

Agricultural, food and other biomass wastes represent an untapped resource of hydrocarbonaceous material with potential for valorisation into fuels and chemical products. A key challenge in this area is directing the characteristics of the products for different applications. Here, the use of different solvents – water and simple alcohols – is shown to directly impact on the products arising from the carbonisation of brewers’ spent grain (BSG). Unprocessed BSG was carbonised using water, methanol, ethanol and 2-propanol under pressure. The synthesised biochar was analysed using infrared spectroscopy, thermogravimetic analysis, elemental analysis and bomb calorimetry. Biochar synthesised in alcohol solvents exhibited significantly different properties to that produced via conventional hydrothermal carbonisation (HTC) in water. HTC yielded a biochar with reduced heteroatom content and more favourable properties for application as a solid fuel. The alcohol solvents yielded greater quantities of water soluble oil (WSO), with the yield increasing with alcohol chain length. These trends are correlated with physical properties such as dielectric constant and increased solubility of organics in higher carbon number alcohols. These results show that the choice of solvent can direct the properties of synthesised chars for specific applications or can maximise the yield of bio-oil for fuel.